Method of determining the drift of a gyrocompass

ABSTRACT

The drift of a directional gyro with a stationary gyro carrier is determined by making a plurality N of measurements of the alignment angle Alpha of the gimbal bearing the gyro rotor with respect to a reference axis determined by the position of the gyro carrier at equal time intervals within a predetermined period of time, producing electrical value signals, preferably pulse sequences, proportional to the measured angular values, and adding the electrical signals to provide a sum which is a measure of the gyro drift. If the alignment angle Alpha 0 at the beginning of the measuring periot T is other than zero, the sum is modified to provide compensation for the initial alignment angle Alpha 0.

United States Patent [72] Inventors Horst Brenda I56] References CitedWalldorf; UNITED STATES PATENTS walmud 3mm"; Funk 3,258,977 7 1966Hoffman 74/5 x N y Germany 3,559,493 2/1971 Brooks, Jr. et al. 74/5 x P3 3,359,805 12/1967 Schlitt 74/5 x [221 Aug-18,1970 3,442,140 5/1969Pelteson.. 235/15025 x 3,319,052 5/l967 Arshal 235/150.25 x v HeiddbeGerman Primary Examiner-Eugene G. Botz [32] Pri rit Au 23,?9 69 yAssistant Examiner-Edward J. Wise 33 Germany Attorney-Spencer & Kaye[31] P19430263 ABSTRACT: The drift of a directional gyro with a stationaW [54] METHOD OF DETERMINING THE D FT OF A gyro carrier is determmed bymaking a plurality N of measure- GYROCOMPASS ments of the alignmentangle a of the g1mbal bearlng the gyro 15 (fl i 5 D i i rotor withrespect to a reference axis determined by the position of the gyrocarrier at equal time intervals within a U-S- p d i d p i d f i p d i gelectrical value 51 l I Cl 3 6 33 signals, preferably pulse sequences,proportional to the meal I! 6g sured angular values and the electricalsignals to P [50] Fleld 0f SCSICI'I vide a sum which is a measure of thegy drift. If the g PHASE SHIFT ZERO 1 renown 1 0110mm; MODZLATORDETEETOR [UL COMPARATOR ment angle a at the beginning of the measuringperiot T is other than zero, the sum is modified to provide compensationfor the initial alignment angle 0 COUNTER MEMORY MEASURING PERIOD eTIMlNG CONTROL ADDING- SUBTRACTING CIRCUIT 7 PAIENTEDMI'I 2 I9113,622,764

MIEEI 2 OF 3 PHASE SHIFT ZERO MODULATOR CROSSING 5 ,POEGE GENERATORDETECTOR m I II \ A GATE COMPARATOR BISTABLE INVERTER CIRCU'T V ICOUNTER I 1 MEMORY MEAsuRING PERIOD TIMING CONTROL CIRCUIT...Y\ I

F: J I? ADDING SUBTRACTING CIRCUIT 7 PULSE 23 GENERATOR 22 Fig. 3

COUNTER Fig. 4

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FROM I7 Horst Brendes I -FORWARDBACKWARD I AK FSnE COUNTER g Q/ATTORNEY;

SHEET 3 [IF 3 DIRECTIONAL GYRO I PHASE SHIFT MODULATOR PULSE GENERATOR.IUL'L\5 GATE 4 ZERO I) x 9 MEASURING PERIOD 25} TIMING CONTROL vCIRCUIT OOIINTER 27 COMPUTER INVENTORS Horst Brendes WcIIIrcIud Bdr EgonFunk BY r %a-ew/ I9 %L ATTORNEYS.

METHOD OF DETERMINING THE DRIFI OF A GYROCOMPASS BACKGROUND OF THEINVENTION The present invention relates to a method for determining thedrift of a directional gyro with a stationary gyro carrier (e.g., astill-standing vehicle) by means of angular measurements.

The fact that directional gyros drift is well known. The drift of adirectional gyro consists of two components, the drift dependent on thegeographical latitude and the random drift inherent to each gyro. With aknown location of use, the drift dependent on the geographical latitudecan be compensated by certain known measures at the gyro. An advancecompensation of the random drift, however, is not possible.

In order to provide compensation for the drift it is therefore necessaryto determine the drift of the gyro at certain intervals (either only therandom drift or the sum of the drifts for an uncompensated gyro) and tomake an extrapolation for the following time period. For this purpose,the gyro carrier, e.g., the vehicle, must be standing still. At a giventime interval the angle which the spin or major axis of the directionalgyro forms with a reference direction is measured. The resulting angulardifference represents the drift during the given time period. If adigital goniometer is available for measuring the angles, which permitsresolution of the measured angular value to one angular unit (I'"")where 640O"" '=360, a measuring time of 3 minutes would result, forexample, in an inaccuracy of the drift determination of 20 per hour.Such an inaccuracy in the drift determination is not tenable in manycases. It is therefore necessary to substantially lengthen the measuringtime, i.e., to increase the time period for the angular measurement, forexample, tenfold. Since the vehicle, however, must remain stationaryduring the drift determination, such a long measuring period is notpractical. Moreover, with the above-mentioned method temporary movementsof the gyro may falsify the measured drift value.

SUMMARY OF THE INVENTION It is therefore the object of the presentinvention to provide a method for determining the drift of a gyro which,with the same resolution capability in the goniometer device utilized,e.g., l' furnishes a greater resolution of the angular value in ashorter measuring period.

It is a further object of the invention to provide a method fordetermining the drift of a gyro wherein temporary movements of the gyrowill not result in a falsification or error in the measuring result orat least will cause only a slight falsification.

This is accomplished, according to the present invention, in that duringa given measuring period T a plurality N of measurements of thealignment angle a of the gimbal bearing the gyro rotor with respect to areference direction given by the position of the gyro carrier are madeat identical time intervals, electrical values are produced which areproportional to these measuring results, these electrical values arealgebrai cally added together to provide a sum representing a measure ofthe drift which may be further processed. If the angle a between thegimbal and the reference direction at the beginning of the measuringperiod is other than zero, this angle a must be considered in themeasurements and the above-mentioned sum modified accordingly.Preferably the values proportional to the goniometer results are pulsesequences, with the number of pulses in each sequence being proportionalto the respective measured value.

In order to determine the drift, the reference direction can be broughtinto coincidence with the alignment of the gimbal (e.g., with the spinaxis of the gyro rotor) at the beginning of the measuring period. Inthis case, the angle a equals 0 and it is sufficient to add the pulsesof the N pulse sequences obtained during the measuring period in theconventional manner. This addition can be made in an electronic counter.This alignment can however be avoided. but in such case as mentionedabove the size of the starting angle a must be taken into consideration.This can easily be accomplished, for example, by measuring the angle aand subtracting or deducting an electrical value corresponding to Nafrom the sum obtained as the measuring result. However, according to afurther feature of the invention it is more favorable, in the event theangle or. is other than zero, to add the pulses of the individual pulsesequences in the first half of the measuring period T to provide a sum,and to to add the pulses of the individual pulse sequences in the secondhalf of the measuring period T to this sum with the opposite sign, i.e.,subtract, so as to form a final sum equal to the difference between therespective sums of the pulses occurring during the two halves of thetotal measuring period T. This operation may be performed by aconventional adding-subtracting arrangement, e.g., as known in thedigital processing art, or by means of a forwardbackward counter whichis controlled to count backward, for example, during the first half ofthe measuring period T and forward during the second half.

According to another feature of the invention, the numerical electricalvalues are passed through an error-value filter prior to the addition inorder to eliminate erroneous measurements produced, e.g., byinterferences.

According to still a further feature of the invention whenever it isdetermined that the angle a to be measured is within a predeterminedrange on either side of the 0/360 angle line, so that during themeasuring period successive measurements may pass over this line andproduce erroneous results, the electrical values corresponding to themeasured angles are modified by an amount corresponding to I.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of gyro drift versustime illustrating the operation of the method according to the inventionfor an initial alignment angle a equal to zero.

FIG. 2 is a graph of gyro drift versus time illustrating the methodaccording to the invention for an initial alignment angle ar which isother than zero.

FIG. 3 is a schematic block diagram illustrating an apparatus forcarrying out the method according to one embodiment of the invention.

FIG. 4 is a schematic block diagram of a portion of the apparatus ofFIG. 3 illustrating a further embodiment of the invention.

FIG. 5 is a schematic block diagram illustrating a further embodiment ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With the aid of FIGS. 1 and 2it will be shown that the measured value produced by the method of thepresent invention represents a parameter or measure of the drift of thegyrocompass. FIGS. 1 and 2 differ from each other in that in the onecase (FIG. 1) the initial or starting angle a at the beginning of themeasuring period T equals zero, while in the other case (FIG. 2) it doesnot equal zero. In both figures it is assumed that the resolution of thegoniometer is 1 and that there is a linear drift course C or C.

As indicated above according to the method of the invention, a pluralityN of angular measurements are taken at fixed intervals within a fixedpredetermined time period T. The graph of FIG. I shows that during theplurality of the measurements taken up to time t, at equal timeintervals, the angular value of the drift registered is 0. Between timet and I, the same number of measurements results in the angular value 1"and thereafter between t and t the angular value 2" results, etc.Between 1., and the end of the measuring period T the angular value 4"is registered for a lesser number of measurements. Since all of themeasurements are made at equal time intervals, the sum of the pulsesobtained in measuring intervals t2, t t .....t.,-T is proportional tothe shaded areas associated with the measuring intervals.

The calculation of the sum F of these shaded areas results in (generallyexpressed):

where n is the number of shaded areas and D the accumulated drift valueat the end of the measuring period T. Since the in creases in height inthe shaded areas are 1" each, n can be replaced by the value for D", andfinally D*=D, if a drift error of l-2""/hour is permissible; resultingin a completed error consideration.

The exact triangular area of the triangle shown in FIG. 1 is Since Tis afixed value, the triangular area is proportional to the drift D to bemeasured. The difference between the exact triangular surface area andthe approximate triangular surface a value which is independent of thedrift D and hence can be considered in the derivation of the desiredmeasuring value. The sum of the pulses added during the measuring periodT thus represents a measure forthe area of the triangle and thus for thedrift D occurring during the measuring period. Due to the summation ofthe pulses of the plurality of measurements, the drift value obtained inthe measuring period can be read out with a higher resolution than 1Additionally, temporary movements of the gyrocompass will besubstantially averaged out with the method of the present invention.

In FIG. 2 it is assumed as shown that the angle at the beginning of themeasuring period a Qln view of the relationships between period Tand thecourse C of the drift, the drift value D during the measuring periodTcan be obtained if the area F, of the shaded parallelogram, or a valuecorresponding to one-half of that area is formed. The area F of theparallelogram is equal to the total area F of the illustrated trapeziumminus twice the area F i.e.:

By determining a value proportional to the area of the paralwith theopposite sign, i.e., a positive sign, to this first negative sum value.By adjusting the number of measurements per unit time, the measuringperiod T itself and the frequency of the pulse sequences in the pulsetrains it is possible to obtain the measured drift value, for example,directly in the angular unit mil and to determine the resolution. It hasbeen found that with the method according to the present inventionmeasuring periods of up to a few minutes (e.g., 3)are sufficient fordetermining the drift value D. In a practical embodiment, the pulsesequence frequency was 3.84 MHz., the number of mea surements per unittime was 600. The shortest measuring period with a resolution of l' forthe goniometer is then 12.8 seconds. The measuring period, however,should be increased by the factor 2" to favorably increase theresolution. Because of the addition and subtraction of the pulsesutilized in this embodiment of the invention, the difference value FF=T/2 mentioned above when discussing the situation where a, equals zeroneed no longer be considered.

Since it is possible that interference in the circuitry or in the gyroproduces individual erroneous measurements, the present inventionfurther provides for the detection and elimination of such erroneousmeasurements through the use of error-value filtering techniques.According to this feature of the invention, instead of the measuredvalue determined to be erroneous being added to the previously addedvalues, the erroneous value is disregarded and the measuring valueobtained from the preceding measurement is again entered into themeasuring result. This is based on the consideration that the individualmeasuring values cannot change suddenly by more than 1' which is theresolution of the goniometer. Details about the possibilities ofconstruction of such an errorvalue filter will become apparent from HG.3 which shows the principles of a circuit arrangement for carrying outthe method of the present invention.

Referring now to FIG. 3 there is shown a directional gyro l which ismounted on a vehicle, and which contains a goniometer in its housing forproducing an output signal dependent on the position of the vehicle withrespect to the alignment of the gyro gimbal. For example. such agoniometer may emit two voltages whose amplitudes correspond to the sineand cosine of the angle between the spin axis of the gyro and thelongitudinal axis of the vehicle. The two output voltages from thegoniometer are fed to a phase modulator circuit 2 which generates twofurther voltages from the first two voltages which correspond in theirphase shift with respect to each other to the measured momentary anglea. The two voltage signals from the circuit 2 are in turn fed to a zerocrossing detector 3 which produces, e.g., at the negative zero passages,start-stop signals for a gate 4 from these two voltages. The gate 4permits pulses from a pulse generator 5 to be fed to an electric counter6 for a period of time corresponding to the phase shifts between the twooutput voltages of circuit 2. With an operating frequency of 600 Hz. forthe voltage applied to the goniometer, the counter 6 is offered 600pulse sequences per second. According to the method of the presentinvention the pulses of these pulse sequences are added and, if angle ais other than zero then the value of a must be taken into consideration.

Various techniques are available to provide compensation for the anglear when it is other than zero. According to the preferred embodiment ofthe method, the individual pulses of the pulse sequences are addedduring the first half of the period T and then algebraically added withthe opposite sign, i.e., subtracted, during the second half of themeasuring period T. In the embodiment of FIG. 3, the desired summationoperation is performed by means of an arrangement including anadding-subtracting circuit 7 and a register 11 whose operation will bemore fully explained below.

However, in order to be able to detect error values produced byinterferences, and to eliminate such values from the drift determinationan error-value filter is connected ahead of the adding and subtractingmechanism 7 which errorvalue filter consists of a comparison circuit 8,a memory 9 and a register 10. After each new determination of the anglea in counter 6, the comparison circuit 8 compares the binary value incounter 6 corresponding to this angle with the binary value stored inmemory 9, which corresponds to the immediately preceding anglemeasurement. After the comparison, circuit 8 causes the new binary valuein counter 6 to be transferred into memory 9. If, as a result of thecomparison, the two binary values are determined to be equivalent, thisvalue is transferred to register 10 whose previously stored value iserased. The binary value newly stored in register 10 is now fed,together with the binary value stored in register 11, and representingthe subtotal, to the adding and subtracting circuit 7 which iscontrolled by the measuring period timing circuitry 17 to add the fed-invalues to a negative number during the first half of the measuringperiod and to add the values fed in during the second half of themeasuring period to this negative number with a positive sign. Theresult of this addition or subtraction is again fed into register 11.The binary value from register l0 fed into adding and subtractingcircuit 7 is immediately newly stored therein. This is necessary inorder to have available a value which can be added or subtracted to thesubtotal of register 11 when the binary values of counter 6 and memory 9are not identical, in which case no new value is fed to register 10.Thus, this error-value filter prevents error values from beingconsidered in the measuring result.

After the measuring period T a binary number corresponding to the driftD to be determined is present in register 11. A digital numbercorresponding to the required resolution of the drift value is stored inmemory 12 after termination of the a measuring period by shifting thecontents of register 11 into memory 12. If this binary value is to beconverted into a pulse width, an electronic counter 13 may be providedto which pulses (e.g., at a 200 Hz. pulse sequence frequency) from pulsegenerator 14 are fed, and another comparison circuit 15 provided betweenthe memory 12 and the counter 13. The output pulse of counter 13occurring when counter 13 changes from its final value to its startingposition is utilized to enable a gate 16. The comparison member 15continuously compares the momentary numerical value of counter 13 withthe value stored in memory 12, and at coincidence emits a pulse whichagain blocks gate 16. The duration of the output pulse of gate 16 isthen a measure for the drift of the gyro. This value can be used tocompensate the drift of the gyro or it can be used for a correctivecalculation of the indicated angle a.

if the pulses of the first half of the measuring period are only addedand the resulting sum is subtracted from the pulses of the second halfof the measuring period, the pulse from counter 13 must block gate 16and the pulse from comparison circuit 15 must enable the gate.

Due to the manner of operation of the error-value filter, a digitalvalue corresponding to angle 01 must be written into register 10 at thebeginning of the measuring period. This value is also written in onlyupon equivalence of two consecutive measuring values.

When at the beginning of the measurement the second equivalence in thecomparison circuit 8 occurs, the comparison circuit 8 opens the gate 18for the starting pulses of detector 3. These pulses are fed to timecontrol circuit 17, which with the first pulse starts the measuringperiod T by activating the adding-subtracting circuit 7. When the sum ofstarting impulses fed in the timing control circuit 17 is equal to halfthe sum of measurements within the time T, a second signal is fed fromcircuit 17 to circuit 7 which causes that the incoming values are nowadded with the opposite sign. At the end of the measuring period T thecircuit 7 is stopped and register 11 is caused to shift its content intomemory 12 (line 19).

When the angle a to be measured is near 0 or near 360, the ol360 linewill likely be passed during the measuring period T. Since the number ofpulses is very small near 0 and very large near 360, such a passage ofthe 0/360 line during the measuring period would effect a completelyerroneous drift measurement. According to a further feature of thepresent invention care is therefore taken before each measuring period,that such a passage over the 0l360 line will not occur. For thispurpose, angle a is determined before each measuring period T and whenthe number of pulses falls below or above a given limit value, theenabling time of gate 4 is extended or shortened, respectively, by avalue corresponding to 180. This is accomplished by negating a voltageapplied to member 3. Angle a is here artificially changed, which,however, has no influence on the measuring result because a iseliminated anyhow.

This must be done before the measuring period T begins. Therefore thefirst equivalence in the comparison circuit 8 prepares a bistablecircuit 20, which is fed with pulses from counter 6. The state of thisbistable circuit 20 is changed when the counter 6 reaches a first giventhreshold and changes again when the counter reaches also a second giventhreshold. Therefore at angles near 0 and near 360 the bistable circuit20 is in the same stable state, in which it causes one of the outputvoltages of circuit 2 to be negated by means of an inverter 2].

In order to be able to register the sum of the pulses up to the end ofthe first half of the measuring period T, register 11 must be providedwith a plurality of binary locations. According to a further feature ofthe present invention, this register may be limited to a number ofbinary locations which is just capable of registering the maximum driftvalue to be expected. Because of the subtraction during the one half ofthe measuring period and addition during the other half the limitationwill have no adverse effect, as becomes apparent from the followingexpression:

the register, etc. The number n indicates how many times X is containedin the two sums of measured values. After the first half of themeasuring period the sum of the measured values minus n)( is registeredin the register. Since X is also reached n times during the second halfof the measuring period, the values nX are cancelled out of the formula.A prerequisite here for a correct indication is that the two sums ofmeasuring values differ by an amount which is less than X.

As indicated above various other techniques for carrying out thesummation operation are available. According to another embodiment ofthe invention, as shown in FIG. 4, the adding-subtracting circuit 7 andregister 11 of FIG. 3 may be replaced by a forward-backward counter 21.The counter 18 is controlled by the period timing control circuit 17 sothat it counts in one direction, preferably the backward direction,during the first half of the measuring period, and in the otherdirection during the second half of the measuring period.

When the error value filter is also provided and therefore also register10 it is additionally necessary to shift the content of register 10 to acounter 22 and to feed to this counter pulses from a pulse generator 23.By these means a pulse sequence is produced corresponding to the contentof register 10 and this sequence is fed to the forward-backward counter21.

According to still a.further embodiment of the invention, in the eventthe initial alignment angle a is other than zero, this initial angle canbe compensated for by modifying the sum of the measured values bysubtracting a value equal to Na,,.

A corresponding embodiment is shown in FIG. 5. in this embodiment theoutput pulses of gate 4 are fed to a counter 24, when the gate 25 isopen. The gate 25 is open during the period T, controlled by timingcontrol circuit 17. The circuit 17 opens also gate 26 feeding the firstpulse sequence within the period T which is proportional to a to thecomputer 27. In the computer 27 Nat is formed and this value issubtracted from the value, which at the end of the period T is fed fromcounter 24 to the computer 27. Therefore the gate 28 is opened at theend of the period T.

It will be understood that the above description of the I,

present invention is susceptible to various modifications, changes andadaptations and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

We claim:

1. A method of determining the drift of a directional gyro with astationary gyro carrier by means of angular measurements comprising, incombination:

within a predetermined period of time T, taking a plurality N ofmeasurements of the alignment angle a of the gimbal bearing the gyrorotor with respect to a reference axis given by the position of the gyrocarrier at equal time intervals;

producing electrical values values;

algebraically adding these electrical values to provide a sum thereofwhich represents a measure of the drift of the gyrocompass; and

modifying said sum to provide compensation for the initial alignmentangle d in the event said angle a is other than zero at the beginning ofthe measuring period T.

2. The method as defined in claim 1 wherein the electrical valuesproportional to the measured values of the angle a are generated aspulse sequences having a number of pulses which is proportional to themeasured values.

proportional to these measured 3. The method as defined in claim 2wherein, when the angle a is other than zero at the beginning of themeasuring period, the sum of the electrical values is modified bydeducting an electrical value equal to Na 4. The method as defined inclaim 2 wherein when the angle a is other than zero at the beginning ofthe measuring period T, the sum of the electrical values is modified byadding the pulses corresponding to the measured values during the secondhalf of the measuring period T to the sum of the pulses produced duringthe first half of the measuring period with the opposite sign so as toform a sum equal to the difference between the respective sums of thepulses produced during the first and second halves of the measuringperiod T.

5. The method as defined in claim 4 wherein said modification of the sumof said pulses is provided by adding said pulses in a forward-backwardcounter which is controlled to count in one direction during the firsthalf of the measuring period T and in the other direction during thesecond half of the measuring period T.

6. The method as defined in claim 2 wherein the numberof measurementsper unit time, the measuring period T and the frequency'of the pulsesequences of the generator producing said electrical pulses are so tunedto one another that the desired resolution in portions of angulardegrees or other units of counting is produced.

7. The method as defined in claim 2 including the additional step offiltering said electrical value signals prior to the summation thereofto eliminate erroneous or error signals.

8. The method as defined in claim 7 wherein said filtering stepcomprises:

storing the number of pulses within each pulse sequence in a firstcounter; comparing the numerical value of the pulses stored in the firstcounter with the numerical value of the preceding measurementstored in amemory;

providing a numerical value signal corresponding to that in said firstcounter for summation purposes when equivalence between the two comparednumerical values results; and

providing a numerical value corresponding to the last previousequivalence comparison for summation purposes when no equivalencebetween the two compared numerical values results.

9. The method as defined in claim 8 wherein the numerical value signalsresulting from the comparison of the signals in the first and secondcounters are provided by:

entering the numerical value signal stored in the first counter into afirst register, for subsequent transmittal to an adding mechanism,whenever the comparison of the contents of the first counter and thememory indicates equivalence; and

retaining a numerical value in the first register equal to that lastentered therein when the comparison of the contents of the first counterand the memory indicates no equivalence.

10. The method as defined in claim 9 wherein the numerical value signalsproportional to the measured values are added by sequentiallytransferring the contents of the first register to an addingarrangement, including a second register, which adds the numericalvalues to provide a sum of the electrical values in the second register;and

wherein, when the angle a is other than zero at the beginning of themeasuring period T, the sum of the electrical values is modified bycontrolling the adding arrangement to add the pulses corresponding tothe measured values during the second half of the measuring period T tothe sum of the pulses produced during the first half of the measuringperiod with the opposite sign so as to form a sum equal to thedifference between the respective sums of the pulses produced during thefirst and second halves of the measuring period T. 11. The method asdefined in claim 10 wherein said step of algebraicall addin theelectrical values to provide a sum includes the s eps of l transferringthe numerical values stored in said first and second registers to anadder, (2) transferring the numerical value corresponding to theresultant sum to the second register, and (3) repeating steps l) and (2)until all of the N numerical values have been added together.

12. The method as defined in claim 10 wherein said second register is aforward-backward counter, and wherein said step of algebraically addingthe electrical values to provide a sum includes sequentiallytransferring the numerical values stored in said first register to saidsecond register until all of the N numerical values have been addedtogether.

13. The me-hod as defined in claim 10 including the further steps of:

transferring the numerical value corresponding to the sum present in thesecond register at the end of the measuring period T to a furthermemory; comparing the numerical content of said further memory with thenumerical content of a counter which is continuously counting a train ofinput pulses to provide a first output signal when equivalence occursbetween the momentary numerical value in the counter and the numericalvalue stored in the further memory;

providing a second output signal when the counter changes from itshighest count position to its lowest count position; and

utilizing said first and second output signals to enable and block agate whereby the pulse width determined by.

opening and closing of the gate is a measure of the drift of thedirectional gyro. 14. The method as defined in claim 2 including theadditional steps of:

prior to the beginning of the measuring period T, determining whetherthe drift angle a to be measured lies in the vicinity of the O/360angular direction; and

changing the length of the pulse sequences corresponding to the measuredangular values by an amount corresponding to 180 if the angle to bemeasured lies within a predetermined range on either side of the 0/360line.

15. The method as defined in claim 9 including the steps of:

prior to the beginning of the measuring period T, measuring themomentary angle a at least twice in succession until equivalence betweentwo successive measured numerical values is determined, and transferringthe measured value at equivalence to the first register.

t I I i t

1. A method of determining the drift of a directional gyro with astationary gyro carrier by means of angular measurements comprising, incombination: within a predetermined period of time T, taking a pluralityN of measurements of the alignment angle Alpha of the gimbal bearing thegyro rotor with respect to a reference axis given by the position of thegyro carrier at equal time intervals; producing electrical valuesproportional to these measured values; algebraically adding theseelectrical values to provide a sum thereof which represents a measure ofthe drift of the gyrocompass; and modifying said sum to providecompensation for the initial alignment angle Alpha 0 in the event saidangle Alpha 0 is other than zero at the beginning of the measuringperiod T.
 2. The method as defined in claim 1 wherein the electricalvalues proportional to the measured values of the angle Alpha aregenerated as pulse sequences having a number of pulses which isproportional to the measured values.
 3. The method as defined in claim 2wherein, when the angle Alpha 0 is other than zero at the beginning ofthe measuring period, the sum of the electrical values is modified bydeducting an electrical value equal to N Alpha
 0. 4. The method asdefined in claim 2 wherein when the angle Alpha 0 is other than zero atthe beginning of the measuring period T, the sum of the electricalvalues is modified by adding the pulses corresponding to the measuredvalues during the second half of the measuring period T to the sum ofthe pulses produced during the first half of the measuring period withthe opposite sign so as to form a sum equal to the difference betweenthe respective sums of the pulses producEd during the first and secondhalves of the measuring period T.
 5. The method as defined in claim 4wherein said modification of the sum of said pulses is provided byadding said pulses in a forward-backward counter which is controlled tocount in one direction during the first half of the measuring period Tand in the other direction during the second half of the measuringperiod T.
 6. The method as defined in claim 2 wherein the number ofmeasurements per unit time, the measuring period T and the frequency ofthe pulse sequences of the generator producing said electrical pulsesare so tuned to one another that the desired resolution in portions ofangular degrees or other units of counting is produced.
 7. The method asdefined in claim 2 including the additional step of filtering saidelectrical value signals prior to the summation thereof to eliminateerroneous or error signals.
 8. The method as defined in claim 7 whereinsaid filtering step comprises: storing the number of pulses within eachpulse sequence in a first counter; comparing the numerical value of thepulses stored in the first counter with the numerical value of thepreceding measurement stored in a memory; providing a numerical valuesignal corresponding to that in said first counter for summationpurposes when equivalence between the two compared numerical valuesresults; and providing a numerical value corresponding to the lastprevious equivalence comparison for summation purposes when noequivalence between the two compared numerical values results.
 9. Themethod as defined in claim 8 wherein the numerical value signalsresulting from the comparison of the signals in the first and secondcounters are provided by: entering the numerical value signal stored inthe first counter into a first register, for subsequent transmittal toan adding mechanism, whenever the comparison of the contents of thefirst counter and the memory indicates equivalence; and retaining anumerical value in the first register equal to that last entered thereinwhen the comparison of the contents of the first counter and the memoryindicates no equivalence.
 10. The method as defined in claim 9 whereinthe numerical value signals proportional to the measured values areadded by sequentially transferring the contents of the first register toan adding arrangement, including a second register, which adds thenumerical values to provide a sum of the electrical values in the secondregister; and wherein, when the angle Alpha 0 is other than zero at thebeginning of the measuring period T, the sum of the electrical values ismodified by controlling the adding arrangement to add the pulsescorresponding to the measured values during the second half of themeasuring period T to the sum of the pulses produced during the firsthalf of the measuring period with the opposite sign so as to form a sumequal to the difference between the respective sums of the pulsesproduced during the first and second halves of the measuring period T.11. The method as defined in claim 10 wherein said step of algebraicallyadding the electrical values to provide a sum includes the steps of (1)transferring the numerical values stored in said first and secondregisters to an adder, (2) transferring the numerical valuecorresponding to the resultant sum to the second register, and (3)repeating steps (1) and (2) until all of the N numerical values havebeen added together.
 12. The method as defined in claim 10 wherein saidsecond register is a forward-backward counter, and wherein said step ofalgebraically adding the electrical values to provide a sum includessequentially transferring the numerical values stored in said firstregister to said second register until all of the N numerical valueshave been added together.
 13. The method as defined in claim 10including the further steps of: transferring the numerical valuecorresponding to the sum presEnt in the second register at the end ofthe measuring period T to a further memory; comparing the numericalcontent of said further memory with the numerical content of a counterwhich is continuously counting a train of input pulses to provide afirst output signal when equivalence occurs between the momentarynumerical value in the counter and the numerical value stored in thefurther memory; providing a second output signal when the counterchanges from its highest count position to its lowest count position;and utilizing said first and second output signals to enable and block agate whereby the pulse width determined by opening and closing of thegate is a measure of the drift of the directional gyro.
 14. The methodas defined in claim 2 including the additional steps of: prior to thebeginning of the measuring period T, determining whether the drift angleAlpha to be measured lies in the vicinity of the 0/360* angulardirection; and changing the length of the pulse sequences correspondingto the measured angular values by an amount corresponding to 180* if theangle to be measured lies within a predetermined range on either side ofthe 0/360* line.
 15. The method as defined in claim 9 including thesteps of: prior to the beginning of the measuring period T, measuringthe momentary angle Alpha at least twice in succession until equivalencebetween two successive measured numerical values is determined, andtransferring the measured value at equivalence to the first register.